The bloodshed came on the heels of a string of attacks in Turkey, blamed on extremists but also on Kurdish rebels.
The bomb aimed at a convoy of military service vehicles, Ankara governor Mehmet Kiliclar said, quoted by the CNN-Turk and NTV channels.
Plumes of smoke rose from the scene, close to the headquarters of the Turkish military and the parliament.
The powerful blast was heard all over the city, sending residents to their balconies in panic, an AFP correspondent said.
The army said the attack took place at 1631 GMT and had targeted "service vehicles carrying army personnel". It did not give a toll.
#Turkey #Update: At least 5 killed, 10 injured after car bomb blast hits military vehicles in #Ankara. https://t.co/oNcdWQDlKN
— José Miguel Sardo (@jmsardo) February 17, 2016
"The terror attack was carried out when the vehicles were waiting for traffic lights at a road junction," it added.
Prime Minister Ahmet Davutoglu cancelled a planned visit to Brussels on Thursday after being briefed, his office said.
Ambulances and fire engines were sent to the scene, which is near the Turkish military headquarters and the parliament. Wounded victims were seen being taken away on stretchers.
NTV television said the explosion happened near a residential block for top-level military staff.
Twin bombs kill 86 at pro-Kurdish rally in Turkish capital
Images from the scene showed fire-fighters trying to overcome a fierce blaze from wrecked service buses.
The spokesman for the ruling Justice and Development Party (AKP), Omer Celik, said on Twitter he strongly condemned the "act of terror".
"But our determination to fight (terror) will become even greater," he added.
Turkish police threw a security cordon around the area. A second blast later rocked the area, the AFP correspondent said, but media said this was police detonating a suspicious package.
There was no immediate indication about who carried out the attack.
The Islamic State group has been blamed for a string of bombings in the country since the middle of last year but the outlawed Kurdistan Workers' Party (PKK) has also killed dozens of soldiers in attacks mainly in the southeast of the country.
Ten Germans killed in Istanbul suicide bombing, Berlin says
The capital was already on alert after 103 people were killed on October 10 when two suicide bombers blew themselves up in a crowd of peace activists in Ankara, the bloodiest attack in the country's modern history.
Eleven people, all German tourists, were also killed on January 16 when a suicide bomber blew himself up in the tourist heart of Istanbul.
Those attacks were blamed on IS militants, as were two other deadly bombings in the country's Kurdish-dominated southeast earlier in the year.
Turkish authorities have in recent weeks detained several suspected IS members, with officials saying they were planning attacks in Istanbul and Ankara.
But Turkey is also waging an all-out assault on the outlawed PKK which has repeatedly attacked members of the security forces with roadside bombings on their convoys in the southeast.
The PKK launched an insurgency against the Turkish state in 1984, initially fighting for Kurdish independence although now more for greater autonomy and rights for the country's largest ethnic minority.
The conflict, which has left tens of thousands of people dead, looked like it could be nearing a resolution until an uneasy truce was shattered in July.
Turkish artillery in southern Turkey shelled positions of Kurdish fighters in Syria for the fifth day in the row on Wednesday in an escalating standoff, reports said.
Turkey says the Syrian Kurdish Democratic Union Party (PYD) and its People's Protection Units (YPG) are merely the Syrian branch of the PKK and themselves terror groups.
Meanwhile the banned ultra-left Revolutionary People's Liberation Party-Front (DHKP-C) has also staged a string of usually small-scale attacks in Istanbul over the last few months.
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